Carbon Nanotube Appliques for Fatigue Crack Diagnostics

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1 Carbon Nanotube Appliques for Fatigue Crack Diagnostics Seth S. Kessler, Ph.D. President/CEO Metis Design Corporation 1 September 2015 structural health monitoring multi-functional materials lean enterprise solutions 205 Portland St Boston, MA

2 Conformal Multi-functional Assemblies Conformal assemblies for composite & metallic host structures Central carbon nanotube (CNT) layer is core to these properties Surrounded by electrically insulating layers (film adhesive and/or GFRP) Selective electrodes integrated to steer current flow No impact to physical structure, µm & 5-10 g/m 2 Can be co-cured with composite laminate Can be installed over composite or metallic skin in secondary process Enable multi-functional capabilities: conducting, heating, sensing IWSHM of 20

3 CNT Structural Health Monitoring SHM improves reliability, safety & reduced costs sensors add weight, power consumption & computational bandwidth cables add weight, complexity, as well as durability & EMI concerns scaling SHM for large-area coverage has presented challenges Advantages of proposed CNT-based SHM methodology sensing elements actually improve specific strength/stiffness of structure conformal electrodes lighter & more durable than cable simple to scale over large structure, maintains good local resolution Traditional SHM CNT-based SHM IWSHM of 20

4 Sparse Electrode Notch-Cutting Tests (N ) Simple notch-cut experiment presented at prior IWSHM in mm² CNT w/160 mm² damage yields ~25% in resistance increase same damage in 1 m long strip of same width would yield ~2% change 10 mm² damage would still be over noise floor 2D network resistor model in good agreement with data IWSHM of 20

5 4-Point Bent Results Under Load (N ) Loaded measurements Unloaded measurements Simple 4-pt bend experiment also presented at IWSHM 2011 Resistance is proportional to strain for low displacement tensile-side resistance increases due to CNT network being stretched-out compressive-side resistance decreases due to CNT being pushed together Permanent resistance increase after 25 mm deflection (>400 N) IWSHM of 20

6 Environmental & Mechanical Studies (N ) To use CNT as sensors in service, need to evaluate durability Environmental effects Mechanical effects Enhanced version of basic 4-point setup from prior tests Rather than static load, used Labview-controlled stepper-motor In-situ monitoring of load, displacement, temp, strain, CNT resistance 1 Hz cycle rate if collecting data, 10 Hz if no data during cycles Same setup used for 3 sets of tests, 3 repetitions for each Monitor resistance with various electrode materials & coatings Monitor resistance with various temperature steps Strain (enforced) Fatigue Creep IWSHM of 20

7 Environmental Testing Results (N ) Poor electrode selection Good electrode combination IWSHM of 20

8 Compensation for (temp.) & (hum.) (N ) IWSHM of 20

9 Automated 4-Point Test Bending Rig IWSHM of 20

10 Enforced Strain Results (N ) IWSHM of 20

11 Fatigue Test Results (N ) IWSHM of 20

12 Creep Test Results (N ) IWSHM of 20

13 CNT based Continuum Crack Gauge (AF ) Targeted detection of flaw growth in known location Addressing fleetwide problems or critical locations Alternative to traditional crack gauge Focus on crack growth in metallic parts for fixed-wing aircraft Proposed CNT solution Small (5x5 cm) CNT patch with electrodes around perimeter Ability to detect fatigue crack, estimate length & orientation Non-contact resistance measurements for difficult to access locations Phase I Study Funded by AFRL, partnered with LMCO JSF Calibrated milled-notch results Demonstrated fatigue crack growth monitoring Demonstrated passive wireless data acquisition IWSHM of 20

14 Continuum Crack Gauge Model (AF ) IWSHM of 20

15 Continuum Crack Gauge Calibration (AF ) Max error ~ 5 mil overprediction Size of single element in model IWSHM of 20

16 Continuum Crack Gauge Experiment (AF ) IWSHM of 20

17 Passive RFID Proof-of-Concept (AF ) IWSHM of 20

18 CNT Continuum Crack Gauge Summary Use of CNT as sensing method previously explored w/navy SBIR Showed strong correlation to tensile/compressive strain loads Clear trends observed for impact, notch & overload damage Durability of approach was investigated Identified/eliminated sources of drift to improve sensitivity/reliability Quantified/compensated for effects of temperature & moisture absorption Explored repeatability under strain, observed no effects of creep & fatigue Demonstrated continuum crack gauge w/afrl SBIR Simple model in close agreement with experimental results Could calibrate crack length & orientation from orthogonal electrodes Grew real fatigue cracks in 3 specimens, accurate crack length predictions Proof-of concept demo measured resistance change with RFID approach IWSHM of 20

19 Future Work (AFRL Phase II SBIR) Task 1: Sensor Optimization. materials & fabrication procedure selection. Downselect installation (including self-curing) Task 2: RFID Hardware Development. Design, fabricate & test send/receive hardware for collecting data and displaying results Task 3: Sensor Calibration & Validation. Conduct several couponscale tests to build calibration table for crack size/orientation Task 4: Initial Probability of Detection Report. Conduct a first pass PoD assessment for the optimized sensor (MIL-HDBK-1823A) Task 5: Durability Assessment. Conduct MIL-STD-810G to determine susceptibility to aircraft environmental conditions Task 6: Blind Demonstration. Working with LMCO demonstrate technique blindly on a large F-35 relevant built-up test article IWSHM of 20

20 Technical & Business Contact Seth S. Kessler, Ph.D. President/CEO Metis Design Corporation x (cell) IWSHM of 20